The recommendation I’ve always followed is as follows:
Move/push the gantry and carriage all the way to one end of the machine, so that you have maximum lengths of “exposed”, uninterrupted X and Y belts.
Using a fish or luggage scale, pull up on the middle of a belt to 1" of deflection. The scale should measure about 4 lbs, plus or minus 0.5 lbs.
Going back to addressing your main issue… Be sure to inspect all of your belts thoroughly. It’s been a few years since I put my machine together, but at the time, Inventables was having a problem with the belts they were supplying in that they were frequently ending up with mauled teeth and even breakage.